On Nov 15, 2011, at 11:50 PM, Jordan Schatz wrote:
> From Racket v5.2 release notes:
>>> Internal-definition expansion has changed to use let* semantics for
>> sequences that contain no back references. This change removes a
>> performance penalty for using internal definitions instead of let in
>> common cases, and it only changes the meaning of programs that capture
>> continuations in internal definitions. Internal definitions are now
>> considered preferable in style to let.
>> I'm not sure that I understand, but if I have it figured out then this:
>> (define (foo x)
> (local [(define i 10)
> (define j 12)]
> (+ x i j)))
>> Is now considered better style then this?
>> (define (foo2 x)
> (let ([i 10]
> [j 12])
> (+ x i j)))
>> Why?
Your question suggests that you come from a teaching language
background where we introduce only local definitions. In ISL
and ISL+lambda, the use of local makes it easier to move global
transformations into a local scope and vice versa. Most importantly,
these movement preserve the exact semantics of the definitions,
including errors.
An *internal* definition differs slightly from a local definition
in syntax and *semantics*:
(define (foo x)
(define i 10)
(define j 12)
(+ i j x))
It's preferable to local defines for aesthetic reasons because it
consumes less horizontal space (a smaller indentation).
-- Matthias